GPR, or ground-penetrating RADAR (where RADAR is “RAdio Detection And Ranging), is a technology used to assess the composition and location of heterogeneous materials. GPR uses common radio frequencies and is particularly useful in that it is both non-destructive and non-ionizing. In fact, GPR uses frequencies similar to a cellular phone, but at far lower power levels. Common applications include locating the precise position of rebar within a concrete wall/floor, identifying and locating buried objects underground, assessing the quality and uniformity of an asphalt or concrete highway surface, and detecting deterioration on bridge decks. In road surface applications, GPR is used, for example, to detect cracks, fissures, or contamination in any one of the chip seal, pavement layers, gravel base, and so forth. In many roadway applications, a resolution of features of the road surface of less than one inch (2.54 cm) is desired. Such systems may be mounted on vehicles, travelling over the surface while acquiring measurement data. GPR systems are disclosed in more detail in U.S. Pat. No. 5,499,029 to Bashforth, et al., and U.S. Pat. No. 5,384,715 to Lytton, which are hereby incorporated by reference.
There are two common types of GPR for road/bridge surface measurement: Ground-coupled and air-launched. Both may operate on an ultra-wideband frequency range. However, in the current state of the art, neither is completely accurate or repeatable in their measurement response for a given scenario. Noise and distortion affect the system and, in fact, the transmit pulse itself has varying degrees of accuracy compared to what is intended to be transmitted. For example, the pulse may last an extra few nanoseconds longer than intended or the present pulse may not have finished reflecting back at the receiver before the next pulse is transmitted, causing distortion or interference. Further, the receiver may introduce additional distortion and time-domain blurring. The amplitude of the pulse at a given subset of the transmission frequency may also vary. Reflections may return at different times due to multipath distortion and clutter.
Presently, the known methods and devices which account for distortion and interference function by calibrating the transmitting signal based on a reflection off a metal plate. Then, the received reflection when reflecting off of a heterogeneous material, such as a road surface, is calculated by subtracting from the result the received reflection off the metal plate. Similarly, distortions due to reflections off of a vehicle on which the antenna is mounted may be subtracted. This, however, is imprecise because the actual signal transmitted may vary with each transmission and, in fact, the actual characteristics of the signal are not known that is, there is a difference between an ideally transmitted signal and the actual transmitted signal. Even when this error is within a few nanoseconds, a noticeable decrease in resolution, in a GPR system, results.
Thus, there remains an unsolved need in the art to provide a GPR device or method which provides, or corrects, for errors in the transmit pulse signal or receipt thereof.